The present invention generally relates to a positive displacement volumetric infusion cassette pump, and more specifically, to a disposable cassette adapted for use with such a pump, the cassette having an elastomeric membrane that is displaced by a plunger into a cavity to pump fluid and including integral inlet and outlet valve components formed on a surface of the elastomeric membrane.
Various types of pumps are used by medical personnel to infuse drugs into a patient""s body. Of these, cassette infusion pumps are often preferred because they provide a more accurately controlled rate and volume of drug infusion than other types of infusion pumps. A cassette pump typically employs a disposable plastic cassette coupled between a proximal liquid line extending from a drug reservoir or source, and a distal fluid line that is coupled to the patient""s body. The cassette is adapted to engage and be driven by a pump mechanism that includes a control and an interface for setting the desired flow rate, volume of fluid, and other parameters that control the infusion process.
In one prior art design of a disposable cassette, the cassette comprises a plastic shell or housing having a facing member joined to a base. The cassette is inserted into an appropriate receptacle in a pump chassis that typically includes a microprocessor controller and a motor or solenoid-actuated driver. A thin elastomeric sheet or membrane is encapsulated between the two sections. Inlet and outlet flapper valves are formed on one side of this elastomeric membrane and seal against adjacent surfaces formed on the base. These flapper valves are actuated in response to fluid pressure and the force applied by a plunger against the elastomeric membrane. As the plunger withdraws following a pumping stroke, the inlet flapper valve opens, enabling liquid to flow from the drug reservoir through an inlet port of the cassette and into a pumping chamber defined in the base and covered by the elastomeric membrane. The plunger actuated by the motor or solenoid in the pump driver displaces the elastomeric membrane into the pumping chamber, opening the outlet flapper valve and forcing liquid under pressure from the pumping chamber, through an outlet port. The pump chassis thus provides the driving force that pumps liquid through the cassette. In addition, the pump chassis normally includes one or more pressure sensors and air bubble sensors for monitoring and controlling the drug infusion process to protect against potential problems that may arise during delivery of a medicinal liquid to a patient.
Cassette infusion pumps have been widely adopted by the medical profession, which uses millions of such disposable cassettes per year. As is common with other high volume production items, manufacturers continually strive to improve their products. For instance, in prototype unit developed by applicant as a replacement for an existing product, it was observed that during operation, the prototype cassette produced a surprising level of audible noise. Much of this undesirable noise can be attributed to the operation of the flapper valves. Also, in prior art cassettes, when exposed to higher than optimal pressure conditions, the flapper valves can be xe2x80x9cblown,xe2x80x9d that is, the flapper valves can be forced beyond their sealing surfaces by excess pressure. It would therefore be desirable to provide a cassette that includes valve elements that produce substantially less audible sound when operating, and are much less susceptible to being xe2x80x9cblown.xe2x80x9d
Improving the reliability of disposable cassettes is also a goal of both end users and manufacturers. As noted above, disposable cassettes frequently are adapted to engage with air or pressure sensors included on the pumping mechanisms used, which can trigger an alarm to alert an operator of an undesirable or unsafe operating condition. The disposable cassettes generally include sensor ports that enable the sensors disposed on the pumping mechanism to monitor a parameter such as pressure or the presence of air in a fluid line. Including the sensor ports within a disposable cassette can increase the size and cost of the cassette. It would be desirable to eliminate any air sensor ports from the disposable cassette, so that the size and cost of disposable cassettes can be substantially reduced. Instead, the air sensors should be positioned to sense parameters relating to the presence of air contained within a tube set that is connected to the outlet port of the cassette. For proper operation, it is important that the positional relationship between the external air sensors and tubing be stable and consistent, because if the tubing moves relative to the sensors, false alarms can be generated, and/or errors in the monitored parameters can result. It would thus be desirable to provide a cassette that incorporates elements, which ensure the fluid tubing remains in a predetermined position relative to external air sensors, to reduce the possibility of erroneous sensor readings and false alarms due to movement of the fluid tubing relative to the sensors.
Another goal in the further development of disposable cassettes is improving the accuracy with which a medicinal liquid is delivered. It is well understood that air bubbles within cassettes are undesirable for several reasons. While gross amounts of air bubbles, such as levels that pose a risk to a patient""s health by causing an embolism, are not much of a risk in such systems, even smaller volumes of air retained within a cassette pumping chamber can adversely impact the accuracy with which medicinal liquid is delivered to a patient. Prior art cassettes typically attempt to prevent air bubbles in the system by using a combination of an integral air trap and appropriate cassette priming procedures. While air traps and proper priming techniques generally avoid the delivery of large volumes of air that can pose a health risk, smaller volumes of air bubbles that become trapped within the cassette are more difficult to remove. When air bubbles are present within the pumping chamber, accuracy is affected in several ways.
The volume of the pumping chamber is a critical parameter in the algorithm controlling the pump to achieve accuracy in delivering a desired volume of medicinal liquid. The presence of air bubbles within the pumping chamber effectively reduces the volume of the pumping chamber, so that less than a desired volume of fluid will be delivered each pump cycle. Increasing the complexity of this problem is that pressure conditions within the pumping chamber vary during the pumping cycle, so that the actual volume of a fixed mass of air within the pumping chamber is not constant. Thus, the actual volume of fluid delivered cannot be accurately determined and compensated, because the volumetric error caused by the mass of air trapped within the pumping chamber is not constant. Accordingly, it would be desirable to provide a disposable cassette including elements that reduce the generation and/or retention of air bubbles within the pumping chamber, and elements that promote the removal of any air bubbles that are present in the pumping chamber.
Another aspect of the accuracy of prior art disposable cassettes relates to the elastomeric membrane. In prior art cassettes, the elastomeric membrane typically requires a break-in period the first time the cassette is used. This break-in period is required to enable the elastomeric membrane to reach an equilibrium, so that the repetitive manipulation of the membrane during successive pumping cycles produces repeatable results. Generally, the break-in period is required to enable the membrane to become seated with respect to the facing member and base of the housing that retain the elastomeric membrane, so that repeated manipulation of the membrane does not result in any further stretching or movement of the membrane relative to the housing.
Clearly, it would be desirable to provide a disposable cassette that reduces operating noise, that provides enhanced protection against valve deformation (failure) under excessive pressure conditions, that substantially reduces false alarms and increases the reliability of external sensor data, that provides increased accuracy by reducing the volume of air trapped in the pumping chamber, and by eliminating the need for a break-in period of the elastomeric membrane. The prior art does not provide such a disposable cassette.
The present invention defines a cassette that is adapted to engage a drive mechanism, for use in infusing a fluid into a patient. The cassette includes a housing having a base on which is mounted a facing member. An elastomeric membrane is secured between the facing member and the base. A fluid path between the elastomeric membrane and the base extends through the cassette between an inlet port and an outlet port. The fluid path includes an inlet passage coupled in fluid communication with the inlet port, an outlet passage coupled in fluid communication with the outlet port, and a pumping chamber disposed between the inlet passage and the outlet passage. When the elastomeric membrane is displaced into the pumping chamber, it is adapted to force a fluid from the pumping chamber. The elastomeric membrane has a substantially T-shaped lip extending around and proximate its peripheral edge. The T-shaped lip is captured in an interference fit within a groove formed in opposed surfaces of the facing member and the base so that the elastomeric membrane is stretched taut as the facing member is joined to the base. The groove includes inclined surfaces that are in interference with corresponding inclined surfaces on the T-shaped lip. When the facing member is seated onto and joined to the base with the lip of the elastomeric membrane captured in the groove, a tension in the elastomeric membrane produced by the interference fit due to an interaction of the inclined surfaces of the groove and the T-shaped lip compensates for any inelastic deformation of the elastomeric membrane occurring when the elastomeric membrane is displaced into the pumping chamber. This compensation thus minimizes errors in achieving a desired volume of a fluid infused by the cassette.
In at least one embodiment, an extent of an area of the elastomeric membrane defined by the inclined surfaces of the T-shaped lip is less than an extent of an area defined in the base and facing member by the inclined surfaces of the groove formed therein. Preferably, the facing member is seated against and ultrasonically welded to the base while the T-shaped lip of the elastomeric membrane is compressed in the interference fit within the groove.
Also, the fluid path preferably further includes an inlet valve disposed between the inlet passage and the pumping chamber, and an outlet valve disposed between the pumping chamber and the outlet passage. The inlet valve includes an inlet valve surface formed in the base, and an inlet valve flap formed on an undersurface of the elastomeric membrane that seats against the inlet valve surface when the inlet valve is closed. Similarly, the outlet valve includes an outlet valve surface formed in the base, and an outlet valve flap formed on the undersurface of the elastomeric membrane that seats against the outlet valve surface when the outlet valve is closed. In at least one embodiment, the inlet valve surface on the base includes a ramp, and the inlet valve flap is substantially thinner at a depending tip thereof than a depending tip of the outlet valve flap. The depending tip of the inlet valve flap is also substantially thinner than a portion of the inlet valve flap where it is joined to the undersurface of the elastomeric membrane. In this manner, the inlet valve and the outlet valve are configured to substantially reduce audible noise produced as the inlet valve and the outlet valve open and close. The thickness of the outlet valve flap and its disposition relative to the outlet valve surface preferably provide a biasing force tending to keep the outlet valve closed until a fluid pressure in the pumping chamber reaches a predetermined level. This biasing force is selected to prevent siphon free flow of a fluid through the cassette.
The base further preferably includes a tube support member that extends beyond the outlet port and is adapted to support a tube that is coupled to the outlet port to receive fluid forced from the cassette. The tube support member ensures that the tube remains statically positioned relative to an air-in-line sensor provided on the drive mechanism during use of the cassette.
Another aspect of the present invention is directed to a method for mounting an elastomeric membrane in a cassette used for infusing a fluid into a patient, so as to pre-load the elastomeric membrane under an outwardly directed tension. The steps of the method include providing a generally T-shaped lip extending around and proximate to a peripheral edge of the elastomeric membrane. The T-shaped lip has inclined surfaces extending from where the T-shaped lip extends from a generally planar surface of the elastomeric membrane toward distal tips of the T-shaped lip. Other steps of the method include providing a base and a facing member for the cassette that each include a groove with inclined surfaces shaped and sized to receive a different distal tip of the T-shaped lip in an interference fit.
The steps of the method further include positioning a distal tip of the T-shaped lip of the elastomeric membrane in the groove formed in one of the base and the facing member, then seating the groove formed in the other of the base and the facing member onto an opposite distal tip of the T-shaped lip, and pressing the base and the facing member toward each other until the T-shaped lip seats within the grooves formed in the base and the facing member. The interference fit between the inclined surfaces of the grooves and the T-shaped lip draws the elastomeric membrane taut under a pre-load tension. The next step involves joining the base and the facing members together while the elastomeric membrane is taut, so that the elastomeric membrane is captured under the pre-load tension between the facing member and the base.
Preferably, the step of joining the base and the facing members involves ultrasonically bonding the facing member to the base, or alternatively, the base and facing member are joined using thermal bonding or adhesive bonding. Also, the step of providing the T-shaped lip preferably includes the step of molding the lip around a peripheral edge of the elastomeric membrane. An area of the elastomeric membrane within the lip is less than an area circumscribed by the grooves formed in both the base and the facing member.
Another aspect of the present invention is directed to a cassette that includes an elastomeric membrane having lobes adapted to sweep air bubbles from the sides of the pumping chamber, thereby minimizing errors introduced by air bubbles disposed within the pumping chamber. As before, the cassette is adapted to engage a drive mechanism and is used for infusing a medicinal liquid into a patient. Also as noted above, the cassette includes a housing having a base on which is mounted a facing member. A fluid path is defined between the elastomeric membrane and the base, and extends through the cassette between an inlet port and an outlet port. The facing member is employed to secure the elastomeric membrane relative to the base, and the facing member is preferably not part of the fluid path. The fluid path includes an inlet passage coupled in fluid communication with the inlet port, an outlet passage coupled in fluid communication with the outlet port, and a pumping chamber disposed between the inlet passage and the outlet passage. When the elastomeric membrane is displaced into the pumping chamber, it forces a fluid from the pumping chamber. The elastomeric membrane has a generally planar undersurface facing toward the base, but this surface includes two lobes of increased thickness disposed above the pumping chamber, at opposite sides thereof. The increased thickness of the lobes extends into the pumping chamber so that when the elastomeric diaphragm is displaced into the pumping chamber by a drive mechanism, the lobes sweep away many air bubbles that may be retained on adjacent walls of the pumping chamber. When displaced into the pumping chamber, the lobes also preferably define a shallow elongate path through the pumping chamber between the lobes, so that a substantial portion of any air contained within the pumping chamber is carried out of the pumping chamber with the medicinal liquid being infused.
Preferably the elastomeric membrane further includes an inlet valve flap and an outlet valve flap, both of which depend from the undersurface of the elastomeric membrane. The inlet valve flap, when closed by a fluid pressure within the pumping chamber, forms a seal against an inlet valve surface formed in the base and disposed between the inlet passage and the pumping chamber. In similar fashion, the outlet valve flap when closed, forms a seal against an outlet valve surface defined in the base and disposed between the pumping chamber and the outlet passage. The inlet valve is preferably configured to be substantially thinner at a tip than where the inlet valve flap joins with a substantially planar portion of the elastomeric membrane and comprises a ramp that is inclined at an angle. The angle is selected so that in cooperation with the inlet valve flap, audible noise caused by repetitive opening and closing of the inlet valve flap relative to the inlet valve surface is substantially reduced. The outlet valve flap is preferably substantially thicker than the inlet valve flap and provides a biasing force tending to maintain the outlet valve flap sealed against the outlet valve surface until a pressure in the pump chamber exceeds a predefined level. Preferably, the predefined level is selected to prevent a siphon induced free flow of fluid through the cassette.
The base of the cassette includes a distal member that extends distally past the outlet port and is adapted to support a tube that is coupled in fluid communication with the outlet port. The support of this distal member minimizes movement of the tube relative to an air-in-line sensor that is included in the drive mechanism.